It has been known that expansion and contraction strain is generated in the direction of the thickness of a piezoelectric element plate made of a material such as lead zirconate titanate (PZT) upon the application of a voltage across electrodes provided on the upper and lower surfaces of the piezoelectric element plate. A stacked type piezoelectric actuator is arranged such that piezoelectric element plates of this type are stacked one on another and connected electrically in parallel to each other to superimpose the quantities of the expansion and contraction strain and to make it possible to derive the displacement in the direction of the stack from an end portion of the stacked type piezoelectric actuator.
As shown in FIG. 1, such a generally used stacked type piezoelectric actuator 1 above is arranged such that a plurality of electrostrictive strain elements each constituted by a single piezoelectric element plate 2 are stacked one on another.
In the piezoelectric element plate 2, strain is caused widthwise as expansion and contraction strain is generated in the direction of thickness. That is, the piezoelectric element plate 2 is contracted in width as the thickness of the piezoelectric element plate 2 expands. Similarly, the piezoelectric element plate 2 is expanded in width as the thickness of the piezoelectric element plate 2 contracts. Thus, a displacement deriving end of the stacked type piezoelectric actuator 1 comes in contact with a no-strain portion 3 including a metal block, a displacement deriving portion, and the like, in which very little strain is generated. Therefore, shearing stress is generated in the contact surface of the piezoelectric element plate 2 as strain is generated widthwise so that the electrode formed on the contact surface of the piezoelectric element plate 2 peels or is worn by the stress.
A stacked type piezoelectric actuator 1 arranged in the manner shown in FIG. 2 has been proposed in Japanese Unexamined Patent Publication No. 86880/85. Strain in the direction of expansion and contraction is proportional to a voltage applied to the electrostrictive strain element. On the other hand, strain in the shearing direction is generated to cancel a change in volume of the stacked type piezoelectric actuator 1 in the upward and downward direction generated as the expansion and contraction strain is generated so as to make the volume of the stacked type piezoelectric actuator 1 constant.
The larger the thickness of the electrostrictive strain element is made, the smaller will be the displacement in the shearing direction. Thus, there has been proposed a stacked type piezoelectric actuator wherein a piezoelectric element plate 2a disposed at a displacement deriving end of the stacked type piezoelectric actuator 1 so as to come in contact with the no-strain portion 3 is made different from the other piezoelectric element plates 2 in that the thickness of the former is larger than the latter so as to reduce the quantity of widthwise strain in the piezoelectric element plate 2a and thereby reduce the shear stress in the piezoelectric element plate 2a.
The arrangement of FIG. 2, however, has a disadvantage in that the number of steps in manufacturing is increased and yield is reduced because it is necessary to provide the piezoelectric element plate 2a at the displacement deriving end separately from the other piezoelectric element plates 2.